1. Field of the Invention
The present invention is directed to a liquid metal lubricated plain bearing, and in particular to a part for such a bearing having a surface which adjoins a bearing surface wetted with liquid metal during operation.
2. Description of the Prior Art
Liquid metal lubricated bearings, for example, are employed in x-ray tubes for bearing the rotating anode and are thereby normally accepted in the inside of the vacuum housing of the x-ray tube. An advantage of such liquid metal lubricated plain bearings is their capability of eliminating the dissipated heat that arises in the region of the rotating anode during the generation of x-rays. Gallium, indium or tin alloys that are liquid at room temperature are usually employed as the liquid metal. One problem given such liquid metal plain bearings is that of assuring an adequately high wettability of the bearing surfaces of the plain bearing parts with the liquid metal. As used herein, "bearing surfaces" means those surfaces of the plain bearing parts between which liquid metal must be situated in order to assure a proper operation of the liquid metal lubricated plain bearing. When an adequate wettability of the bearing surfaces is not assured, the liquid metal lubricated plain bearing cannot be completely filled with liquid metal. It is thus then not assured that the bearing surfaces will be wetted completely with liquid metal, leading to a reduction in the supporting capability of the liquid metal plain bearing. Given inadequate wettability of the bearing surfaces, moreover, there is the risk that liquid metal will run out during operation of the bearing surface because the liquid metal film tears off from the bearing surfaces. Additionally, liquid metals are in part extremely reactive, so that they may not wet bearing surface under certain circumstances but will react with the material of the plain bearing part in the region of the bearing surface (gallium is especially highly reactive).
German OS 28 52 908 discloses a liquid metal lubricated plain bearing for the rotating anode of an x-ray tube. The plain bearing parts thereof are composed of molybdenum, because of the good vacuum compatibility of this material and because of the wetting angle of contact between the liquid metal and an oxide-free bearing surface of molybdenum that lies relatively favorable at approximately 100.degree.. Since a thin oxide film forms on bearing surfaces of molybdenum when stored in air or in an atmosphere that contains water vapor, this thin oxide film deteriorating the wettability of the bearing surface with liquid metal, the bearing surfaces are annealed at temperatures of approximately 800.degree. C. in hydrogen or in an atmosphere that contains hydrogen. Subsequently, the parts are stored in a reducing atmosphere until they are employed in order to prevent a renewed occurrence of an oxide layer. A disadvantage is that one is restricted to a specific material for the plain bearing parts.
German OS 38 42 034 and Austrian Patent 361 088 likewise disclose the use of molybdenum as the raw material for the plain bearing part of a liquid metal lubricated plain bearing. Austrian Patent 361 088 also cites tungsten as a suitable material.
A different approach is used in a liquid metal lubricated plain bearing for the rotating anode of an x-ray tube disclosed in European Application 0 479 197. In that bearing, the wettability of the bearing surfaces is produced by providing the plain bearing parts with a coating composed of a carbide, boride or nitride in the region of the bearing surfaces. This is achieved, for example, by coating the plain bearing parts at their bearing surfaces coated using a PVD method (physical vapor deposition, i.e., physical layer deposition in a vacuum). Virtually arbitrary materials can thus be employed for the plain bearing parts, insofar as it is merely assured that these can be provided with a firmly adhering carbide, boride or nitride layer. It has turned out, however, that such liquid metal lubricated plain bearings fall short of expectations in view of their capability of eliminating dissipated heat from the rotating anode.